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Despite all this, one rule has held true for the genitalia of thousands of species&colon; it’s the male that has a penis. But even that rule has been broken. In four closely related insect species, the females have penises and the males vaginas. As a direct result, the females are in charge, too.

Cave-dwellers

In 2010, cave specialist Rodrigo Ferreira of the Federal University of Lavras in Brazil collected 4-millimetre-long insects from caves in the east of the country. He sent them to Charles Lienhard of the Museum of the City of Geneva in Switzerland, and the two realised that the insects were unknown to science. They belonged to a large group called Psocoptera, which also contains the booklice, but they were a new genus. Ferreira and Lienhard called it Neotrogla.

Right from the start, it was obvious these were unusual insects. During copulation, the female was always on top. Lienhard saw that the females had a penis-like organ which he dubbed the “gynosome”, and which they inserted deep into the male.

Intrigued, he brought in Kazunori Yoshizawa of Hokkaido University in Sapporo, Japan to take a closer look. The gynosome is erectile, says Yoshizawa, and it is also a bit prickly. In three of the four species, it was partly covered in spines, while in the fourth it had small bristles. The males have a vagina-like chamber with pouches in its walls, and Yoshizawa realised that the spines fit into them.

Yoshizawa thinks the spines act as anchors, holding the male still while the female mates with him. He tried pulling apart a copulating pair, and wound up ripping the male in two&colon; the female stayed attached. That may explain why he hasn’t yet seen a male resist mating&colon; it would damage itself.

Unlike the classical penis, the gynosome does not squirt out anything. Instead it acts like a vacuum cleaner, sucking up the male’s sperm so that the female can use it to fertilise her eggs. Copulation can last up to 70 hours, non-stop.

Nevertheless the Neotrogla insects are clearly an oddity. That reflects the nature of evolution, says Yoshizawa. “Usually a new structure evolves as a modification of a previously existing structure,” he says. But the female penis is completely novel and thus it is more difficult to evolve one. “Evolution of such novelties is exceptionally rare,” says Yoshizawa.

Why did the Neotrogla females evolve penises? The insects’ strange genitals may be a by-product of where they live&colon; barren caves with little food except dead bats and bat guano. When Yoshizawa dissected the males, he found large packets called spermatophores that seemed to contain not just sperm, but also nutrients. These probably represent a significant food source for the females.

In most species males compete for access to females, but in these insects things may be the other way round – because males are a good source of food. That would explain why the females have evolved gynosomes – to suck food out of the males. It could also explain the anchors&colon; the females don’t want to let the male go until they’ve had a good meal.